This disclosure relates generally to the field of forming shaped thermoplastic articles.
Formation of shaped articles from thermoplastic materials is well known. A wide variety of methods (e.g., thermoforming, casting, molding, and spinning) can be used to confer shape to a molten thermoplastic or to a preformed thermoplastic sheet that has been softened or melted.
Trimming of waste material from one or more edges of a shaped article is a common finishing technique, but leaves a sharp edge that can injure flesh or tear or cut materials which come into contact with the edge. One common use for shaped thermoplastics is to form containers that can be sealed with thin plastic films, such as trays, bowls, or bins intended to contain foodstuffs and intended to be sealed with transparent plastic film. Another common use is to contain items and to seal them from moisture or other materials which may come into contact with the container. Sealing of such containers typically involves extending or stretching the film across a compartment formed in the container and sealing the film around the periphery of the compartment, which periphery is often situated adjacent a trimmed edge of the article that includes the compartment. If that edge is sharp, it can cut or break the film, interfering with the sealing process.
Three well-known sealing technologies are commonly used in sealing foods and foodstuffs to form containers for commercial shipping, storage, display, and sale. These are referred to herein as OW, VSP, and MAP technologies. All of these technologies involve combining a container and a thin plastic film. Owing to the fragility of such films and the need, in many instances, to minimize or eliminate punctures and tears from film portions which serve to define (together with the container) sealed compartments, it is critical to minimize the opportunities for containers to tear, puncture, or abrade the film of the same or nearby containers. In addition to plastic films used for sealing such containers, plastic films are also employed for shipping the containers, such as the “mother bags” (i.e., typically thin plastic bags) used for containing multiple product-in-container-packages during shipping and the plastic grocery bags used by consumers to transport purchased goods from a retailer. This can be achieved by reducing or eliminating sharp or rough container edges, at least positions on the container at which such edges might reasonably contact the film during packaging, storage, shipment, or display.
Overwrap (OW) technology involves enveloping or wrapping a shaped article (e.g., a thermoformed tray, sheet, bowl, or multi-compartment container) with a thin (often transparent) plastic film after a foodstuff or other item has been placed on one or more faces of the article and thereafter sealing the film to itself (e.g., by heating overlapping portions of the film). In such OW technology, sharp or rough edges of the shaped article can cut, abrade, or puncture the film, potentially allowing materials to pass through the film and defeating one or more of its purposes. Heretofore, OW technology has been used primarily together with foamed trays or bins lacking sharp edges. Many municipal recycling schemes exclude or disfavor foamed plastics, and such materials are therefore increasingly disfavored by consumers. It would be desirable if a thermoformable plastic container suitable for use with multiple wrapping technologies, including OW technology, could be made, since thermoformable materials tend to be widely acceptable in recycling programs.
Vacuum-sealed package (VSP) technology involves adhering a thin (again, often transparent) plastic film against a face of a shaped article bearing a foodstuff (for example, or a moisture-sensitive object as an alternate example) on a face of the shaped article. When VSP technology is employed, that item(s) to be packaged are placed on a surface or within a cavity of the shaped article, the film is overlain such that the item(s) are interposed between the shaped article and the film, air (or whatever other gases may be present) is withdrawn from the space between the film and the shaped article (optionally in coordination of application of positive pressure to the exterior of one or both of the film and the shaped article) so that the film is closely opposed against the surface of the shaped article and/or the item(s), and the film is sealed (e.g., by way of an interposed adhesive, through heat-induced adhesion, or by static charge adhesion) to the surface of the shaped article across the desired area (usually completely encircling the item(s)), and any excess film can be trimmed from the desired area. The seal can be resistant to gas flow in order to maintain the gas-evacuated state on the interior of the sealed container. The resulting VSP-sealed package typically has a topology that mimics the shape of the surface of the shaped article having the item(s) thereon.
MAP is an abbreviation for modified atmosphere packaging and refers to a sealing technology in which a flexible (often transparent) film is sealed (e.g., using heat or an adhesive) about the perimeter of a substantially rigid shaped article. When the shaped article is otherwise closed (i.e., when it has no other openings than that sealed by the film), the gases present within the container can be controlled at the time the film is sealed to the article. Thus, if the article and film are sealed in the presence of a selected atmosphere (e.g., a gas, such as one selected to exclude oxygen or to promote fruit ripening), the selected atmosphere can be maintained within the sealed MAP package during subsequent storage, shipping, and display of the package.
As is known in the art, the shaped articles used in OW, VSP, and MAP sealing processes tend to have a variety of industry-accepted geometric shapes and properties which differ among the three types, such that a shaped article useful in one type of sealing process is often poorly suited for use in one or both of the others.
Containers used for OW-sealing, for example, tend to be rectangular and tray- or sheet-shaped, with smooth, blunt edges and rounded corners. The lack of sharp, rough, or pointed edges or corners serves to reduce the likelihood that the film used for overwrapping the container will be torn or punctured upon wrapping. OW-containers often have a flat portion (e.g., on the “bottom” of the container, relative to its intended display configuration) at which the overwrapped film can be urged against itself for the purpose of sealing the film to itself (e.g., upon application of heat to the overlapping film portions sufficient to cause such sealing), thus enclosing the container and any items on or in it.
Containers used for VSP-sealing tend to have a face or surface (sometimes within a concavity) adapted to carry an item to be sealed between the film and the container and adapted to receive the sealing film by virtue of the absence of sharp points, protrusions, or edges. The absence of such features reduces the likelihood of punctures or tears in the film as it is drawn against the surface. Unlike OW-containers, VSP-containers can have sharp edges, corners, or protrusions, at least at portions other than the film-receiving surface, because those portions need not contact the film during sealing. However, such sharp portions can still damage sealing films, especially when multiple VSP-sealed packages are stored, shipped, or displayed near one another, because a sharp portion of one container can damage the film of another container (or a film or tissue in the vicinity of the container).
Containers used for MAP-sealing tend to have a planar surface (e.g., a broad, flat rim) surrounding an opening to be sealed by applying a sheet of film against it, sealing the film to the surface (often substantially irreversibly), and then trimming the film about the perimeter of the seal. Such containers must be configured such that the film can be applied to the surface without substantial risk of tears or punctures before and during sealing and to facilitate trimming of the film from the sealed container. However, because the film typically contacts only limited portions of MAP-containers during the sealing process, MAP-containers can, and often do, include sharp, pointed, or abrasive features at positions not involved in the sealing process.
It would be beneficial if the sharp edges of shaped thermoplastic articles could be displaced in such a way that the risk of injury or damage to sealing films could be reduced. It would be further beneficial if such individual shaped articles could be used with multiple known sealing technologies, such as two or more of OW, VSP, and MAP technologies.
Reducing the sharpness and tendency of thermoformed articles to induce damage and injuries would be advantageous even in the absence of sealing. The subject matter disclosed herein addresses this shortcoming of prior shaped thermoplastic articles.
Others have recognized the desirability of reducing the occurrence of sharp edges at the edges of trays to be overwrapped. For example, Nelson et al. (U.S. patent application publication number 2015/0001127) disclosed a packaging tray that is formed by thermoforming a film sheet to yield a precursor tray having a generally U-shaped flange about its periphery, the open end of the U facing the sealing surface and the peripheral edge of the tray jutting peripherally. Nelson's tray is made by cutting the precursor tray from a sheet of thermoformable material to yield an end in which the peripherally-extending peripheral edge is positioned at the terminus of the outer (relative to the tray body) arm of the U. Nelson then compresses that outer arm inwardly toward the tray body, leaving a smoother crimped portion of the U-shaped flange at the periphery of the tray, with the still-potentially-sharp peripheral edge extending toward the sealing surface. In this way, Nelson et al., generates a tray said to be suitable for overwrapping, an overwrapping film being intended to urge the outer arm of the U-shaped flange toward or against the inner arm. However, because the tray retains a sharp peripheral edge in a position in which it can cut films (the peripheral edge of Nelson's trays can be seen to contact both the film overwrap and the film overwrapping an adjacent tray in Nelson's FIG. 13A, for example), Nelson's tray remains unsuitable for all OW applications and generally unsuitable for use with VSP and MAP technologies. It would be preferable that a tray used for VSP and/or MAP packaging lacks both a sharp peripheral edge of the material from which the tray is formed (see, e.g., peripheral edge 120 in Nelson's FIG. 12A) and any relatively sharp crimp (see item 124 in Nelson's FIG. 12A), because such edges and crimps are capable of snagging, puncturing, or tearing films, either during or after the sealing process.
The subject matter disclosed herein includes shaped thermoformed articles which are suitable for sealing with multiple technologies.
Thermoformed drinking cups having smooth, rolled edges are also known. Such cups are made by thermoforming cups having a flange about the perimeter of the cup opening, the flange including a potentially sharp peripheral edge at the flange end distal to the interior of the cup. The flanged cups are stacked in a nested fashion, heated at their flange portions, and then passed through helical rim-rolling threads to create the rolled edge. Such technology is useful only for rolling the edge surrounding a circular orifice, and is therefore of no practical use in making shaped articles having rolled edges surrounding non-circular openings. Rolled-edge drinking cups are also not designed to facilitate wrapping or sealing with thin plastic films.